The present invention relates to a method for automatically controlling the lateral dynamics of a vehicle with front-axle steering, and more particularly to control taking place as a function of deviations of measured actual values of the yaw velocity from continuously generated desired values of the yaw velocity, and the adjusting of the actual values by setting wheel steering angles (xcex4i) and/or setting wheel braking forces (Pi).
DE 42 26 746 describes a method in which the steering angles of the front wheels of the vehicle are determined as a function of the driving situation. This determination takes place while taking into account the yaw velocity {dot over ("psgr")} of the vehicle as well as the sideslip angle xcex2. In addition to influencing of the lateral dynamics of the vehicle by the steering angle adjustment, a brake intervention can also take place at individual vehicle wheels. The yaw behavior of the vehicle as well as the sideslip angle are used for determining the wheel steering angle and/or braking power variables.
A disadvantage of the known method is that, because of a permanent limitation of the sideslip angle and, as a result, also of the slip angle in numerous driving situations, the maximal lateral control force of the rear wheels of the vehicle cannot be utilized and deviations of the path curve of the vehicle therefore occur from the desired curve corresponding to the driver""s wish even if there is not yet any swerving tendency of the vehicle, and a limited drifting of the vehiclexe2x80x94a relatively large slip angle at the rear axlexe2x80x94could be permitted.
DE 43 25 413 Al describes, a method for determining the sideslip angle and influencing, as a function of this sideslip angle, the lateral dynamics of the vehicle by a steering and braking intervention. Thereby, a swerving of the vehicle can largely be avoided. It was found, however, that an automatic lateral dynamics control alone as a function of the sideslip angle is not sufficient in order to avoid a swerving of the vehicle in all statistically significant situations.
An object of the invention is, therefore, to provide a method which prevents a swerving of the vehicle with high reliability regardless of an extensive freedom with respect to the design of the vehicle reactions upon the setting of the driver""s wish with respect to the course of the vehicle.
This object has been achieved by a method in which by way of a linear vehicle model, which represents a dynamic wish behavior, a first desired value "psgr"des(xcex4, vx) is determined for the yaw velocity "psgr", which desired value corresponds to the driver""s wish with respect to the path movement of the vehicle set by the operation of a steering element (steering wheel, joystick, or the like), in that, under the secondary condition, that the sideslip angle xcex2h in the area of the unsteered rear wheels of the vehicle should not exceed a limited value, another desired value "psgr"des(xcex2) is determined, and in that the smaller of the two desired values with respect to the amount is used as the desired value of the yaw velocity relevant for the automatic control.
Accordingly, while processing quantities which represent the movement condition of the vehicle, particularly the steering angle xcex4 defined by the driver and the vehicle longitudinal velocity vx, by way of a linearized vehicle model, a first desired value {dot over ("psgr")}des (xcex4, vx) for the yaw velocity {dot over ("psgr")} is determined.
This value corresponds to the driver""s wish with respect to the path movement of the vehicle set by the operation of a steering element or device (e.g., a steering wheel, joystick, or the like).
Furthermore, under the secondary condition that the sideslip angle xcex2h in the area of the unsteered rear wheels of the vehicle should not exceed a limited value, another desired value {dot over ("psgr")}des(xcex2h; xcex1h) is determined, and the respective smaller value of the two desired values is supplied to the {dot over ("psgr")}-controller of the automatic control device as a desired value input. Thereby, the sideslip-angle-dependent or slip-angle-dependent automatic yaw velocity control does not become effective before a swerving tendency occurs at the vehicle which can be recognized by the fact that xe2x80x9cnon-physicallyxe2x80x9d high actual values of the yaw velocity occur while the values of the lateral vehicle acceleration are simultaneously low.
In addition, i.e., as long as a swerving tendency does not exist, the automatic lateral dynamics control can take place by way of the {dot over ("psgr")}xe2x80x94desired value output of the vehicle model with a characteristic which, in principle, is arbitrary and corresponds to a desired vehicle handling; for example, an xe2x80x9ceasilyxe2x80x9d oversteering or understeering or neutral vehicle handling.
In a currently preferred implementation of the method according to the present invention, an uncoupling of the sideslip dynamics at the rear axle from those at the front axle corresponds to a type of desired-value formation provided for the control mode of the control device which limits the sideslip angle, at least if the moment of inertia Jz of the vehicle about its normal axis in a good approximation corresponds to the value Jz=mxc2x7lvxc2x7lh. A control mode is achieved which can be implemented in a particularly simple manner and prevents a swerving of the vehicle.
The sideslip and slip angle limit value, which must not be exceeded in the swerve-preventing control mode, can be defined as a fixed value and is then expediently selected corresponding to that value at which the highest possible lateral control force transmission capability of the vehicle rear wheels exists.
For taking into account particularly the road condition, it may also be expedient to determine the sideslip angle limit value from the estimated values {circumflex over (v)}x and {circumflex over (xcexc)} of the vehicle velocity and vx and of the adhesion coefficient xcexc.
For determining the actual value of the sideslip angle xcex2h required for the automatic control, estimated values {circumflex over (v)}y and {circumflex over (v)}x of the lateral vehicle velocity vy and of the longitudinal vehicle velocity vx are suitable, which can be obtained, for example, as outputs of a Kalman filter, as disclosed in DE 43 25 413 A1, as well as measured values of the vehicle yaw velocity.